A polyimide, Matrimid.RTM., has been discovered to form good membranes for the low temperature separation of low molecular weight organic materials from solvents by hyperfiltration. The low molecular weight organic materials can have an average molecular weight of less than 500, while the solvents have a molecular weight of about 100. The separation is accomplished by contacting a liquid feed stream with the dense, active layer of a membrane, which is made from a specific asymmetric polyimide, and recovering the permeate, which is high purity solvent. This separation has particular utility in the industrial process for removal of wax from lube oil by use of solvents.
The lube oil is dewaxed using conventional solvent dewaxing techniques to produce a lube oil feedstream from which the remaining solvent must be removed. In this context, lube oil is a refinery feed stream which may contain a broad range of organic compounds. Although this application is written in terms of the specific first application of the invention, one of ordinary skill in the art will readily recognize that the invention can be used in other contexts, such as the processing of vegetable oils.
There are a number of alternative processes for solvent dewaxing of lube oil. For example, the waxy oil can be mixed with a quantity of warm solvent to form a mixture which is then cooled down to a temperature low enough for wax crystals to form by indirect heat exchange means, and then filtered.
Alternatively, the waxy oil can be directly contacted with volumes of cold dewaxing solvent to lower the temperature of the overall mixture.
Also, the waxy oil can be directly contacted with incremental volumes of cold solvent under conditions of high agitation at multiple stages in a staged chilling column or tower to reduce the temperature of the overall oil/solvent mixture low enough to precipitate at least a portion of the wax. Chilling may be conducted either entirely in the staged agitated chilling tower, or completed in other known ways.
A dewaxing process especially adapted for use in conjunction with the present invention is disclosed in a copending application "Lubricating Oil using Membrane Separation of Cold Solvent from Dewaxed Oil" by R. M. Gould, filed of even date.
Typical dewaxing solvents include ketones having from 3-6 carbons, such as acetone, methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK); C.sub.6 -C.sub.9 aromatic hydrocarbons such as toluene, xylene, benzene; mixtures of ketone with aromatics such as MEK with toluene; halogenated C.sub.1 -C.sub.4 hydrocarbons such as methylene chloride, dichloroethane and mixtures thereof. Dewaxing solvents of choice are the ketones, and mixtures of ketones and aromatics such as MEK with MIBK or toluene.
Regardless of the procedure employed, once the waxy mixture is chilled to the wax separation temperature the precipitated wax is separated from the wax mixture via conventional separation procedures which typically include by way of illustration, filtration and centrifugation.
The dewaxed oil feed stream must now be separated into its respective oil and solvent components. The recovered oil will be used either directly or sent on for further processing and the recovered solvent will be recycled to the dewaxing process.
Solvent is normally separated from oil by distillation. Since solvent dewaxing typically employs solvent to oil ratios of anywhere from 1:1 to 10:1, more typically 3:1 to 6:1 (depending on the technique employed and the type of oil being processed) this means that tremendous volumes of liquids must be processed. Distillation, by its very nature, is an energy intensive operation. Selective permeation of the solvent through a membrane barrier is a technique which ought to reduce energy consumption and processing costs.
While membrane permeation in concept offers processing advantages for the treatment of lube oil, no commercial process yet exists. This type of separation is demanding because the membrane material must be resistant to both the solvent and the oil, and must be capable of effecting the separation. In this case, the chemical character of the molecules to be separated is similar because they are mutually soluble compounds of relatively low molecular weight. But since they differ slightly in molecular weight, the separation is made mainly on the basis of molecular size. Further, the low molecular weight organic materials can have an average weight of less than 500, often in the range of 300 to 400, while the solvents have a molecular weight of about 100. Also, the separation must occur at a high enough rate and yield to be economically competitive for commercial uses. A flow rate of at least about 2 gallons per square foot per day (GSFD) is required, preferably 4 or more and even more preferably about 6-12 GSFD. This flow rate must be combined with a yield, in terms of rejection of the low molecular weight organic (non-solvent) component, of at least about 80%, preferably greater than 90%, and even more preferably, greater than 95% rejection. In addition, as a practical matter, the separation is run at low temperatures, typically +25.degree. to -25.degree. F., which can cause a precipitous drop in membrane efficiency.
A variety of membranes have been proposed for the separation of solvent from lube oil. U.S. Pat. Nos. 4,541,972 and 4,678,555 issued to Wernick Sep. 17, 1985 and Jul. 7, 1987 respectively, disclose a cellulose acetate membrane for the separation of solvent from lube oil; U.S. Pat. Nos. 4,985,138 issued Jan. 15, 1991, 5,093,002 issued Mar. 3, 1992, and 5,102,551 issued Apr. 7, 1992, all to Pasternak, claim various composite membrane structures; U.S. Pat. No. 4,748,288 issued to Bitter et al. May 31, 1988 claims a dense membrane with a halogen substituted silicon compound; European patent application no. 427,338 A1 to Rujkens and Werner claims a plasma treated fluorosilicone membrane; European patent application nos. 460,769 A1 and 460,770 A1 to Marinus and Tinnemans relate to composite membranes from interfacial polymerization and describe the separation of n-docosane from solvent; and U.S. Pat. No. 4,715,960, issued to Thompson Dec. 29, 1987, discloses the use of polycarbonates.
Polyimide-type membranes have been proposed for use for the separation of solvent from lube oil. U.S. Pat. No. 4,532,041 issued to Shuey et al. Jul. 30, 1985 relates to a membrane made from a polyimide for the separation of solvent from lube oil. The polyimide is a copolymer derived from the cocondensation of benzophenone 3,3',4,4'-tetracarboxylic acid dianhydride (BTDA) and a mixture of di(4aminophenyl)methane and toluene diamine or the corresponding diisocyanates, 4,4'methylenebis(phenyl/isocyanate) and toluene diisocyanate, which is available as Upjohn Company's 2080D. None of the experimental results reported in this patent were based on low temperature operation.
U.S. Pat. No. 5,133,867, issued to La Freniere Jul. 28, 1992 discusses the use of the same polyimide as that disclosed in the Shuey patent, above, for recovery of C.sub.3 -C.sub.6 aliphatic hydrocarbons from hydrocarbon oils at low temperature.
Polyimide ultrafiltration membranes have been used to recover dewaxing aids from lube oil, see U.S. Pat. No. 4,836,927, issued to Wan Jun. 6, 1989. These dewaxing aids are high molecular weight polymers, and, in practice, the separation occurs at temperatures of about 70.degree. C. to 250.degree. C. Solvent is not separated. Further development of this concept is disclosed in U.S. Pat. Nos. 4,908,134 and 4,963,303, issued to Anderson on Mar. 13 and Oct. 16, 1990 respectively. These references disclose further refinement of the method of making the polyimide membranes, principally by use of the polyimide used in the present invention, and a pore former. The claimed membranes cannot be used with ketone-containing materials as fabricated.
U.S. Pat. No. 5,067,970, issued to Wang et al. Nov. 26, 1991, relates to an asymmetric membrane prepared from the polyimide of the present invention which is used for a gas separation, that is, of carbon dioxide from methane. Separation of liquids such as solvent from lube oil was not disclosed or suggested, and as can be seen from Example 1 as compared to Examples 2 to 6 below, the present invention shows greater utility for the liquid separation.
European Patent Publication No. 125 907 published Nov. 21, 1984 discusses the use of asymmetric polyimides for separation of dewaxing solvent from dewaxed oil in a lube oil dewaxing process. In Example 2, the polyimide of the present invention was tested, and characterized as "unsuitable for use".